1. Field of the Invention
The present invention relates generally to the field of heat exchangers, more particularly to submersible heat exchangers, and still more particularly to submersible oil heat exchangers.
2. Background Discussion
Many types of heat exchangers are in general use. Included are heat exchangers used in refrigeration and air conditioning systems and water-filled heat exchangers (i.e., radiators) used for cooling water-cooled internal combustion engines. Further included are air and water-cooled heat exchangers, which are used, for example, in conjunction with vehicle, boat and aircraft engine oil and transmission fluid systems.
Oil heat exchangers (including engine oil and transmission fluid heat exchangers, to which the present invention is principally addressed) may be of non-submersible or submersible types. Nonsubmersible oil coolers ordinarily depend upon a cross flow of air to cool the oil flowing through the heat exchanger. Such air-cooled heat exchangers usually require the heat exchangers to be moving rapidly through or relative to the airflow for efficient oil cooling. In contrast, submersible oil coolers depend upon a flow of another fluid, typically water, to cool oil flowing through the heat exchanger. As an example of submersible heat exchangers, many vehicles employ a transmission fluid heat exchanger installed within the engine radiator. The engine-driven water pump circulates radiator water across the oil heat exchanger to cool oil flowing through the heat exchanger; the water is, in turn, cooled by air flowing between coils of the radiator.
Submersible heat exchangers may comprise a stack of relatively flat tubes. A fluid inlet end of each tube is connected to a fluid inlet fitting or manifold and a fluid outlet end of each tube is connected to a fluid outlet fitting or manifold. The tubes are separated by spacers of some type at a distance apart sufficient for enabling an efficient cooling flow of, for example, cooling water, to flow over, across, and around the heat exchanger tubes.
Of course, for effective and efficient transfer of heat from the fluid flowing through the heat exchanger and the flow of cooling fluid, such as water, the material used for construction of the heat exchanger tubes should have a high thermal conductivity.
Submersible heat exchangers, especially oil heat exchangers, commonly utilize oil flow tubes made from stainless steel, which have reasonably good thermal conductivity as well as resistance to corrosion by cooling water. Moreover, the fabrication assembly of stainless steel tubes and inlet and outlet fittings is comparatively easy by use of known brazing processes.
Submersible heat exchangers made from aluminum alloy would have better heat transfer characteristics than those made from stainless steel and would be both lighter in weight and be much less costly material-wise than stainless steel heat exchangers. However, assembly of aluminum alloy heat exchangers has heretofore been very difficult because the assembly brazing process typically used has resulted in the burning through of some of the aluminum alloy tubes used. The resulting holes in the heat exchanger tubes permit fluid flowing through the tubes to leak into the cooling fluid and cooling fluid may leak into the fluid being cooled, thereby contaminating it.
It is, therefore, a principal objective of the present invention to provide a method of constructing an aluminum alloy, submersible heat exchanger that enables the use of a conventional aluminum brazing process while eliminating the burn-through defects of any of the aluminum alloy heat exchanger tubes. Another principal objective is to provide an aluminum alloy heat exchanger made by such a construction method.
In accordance with the present invention there is provided a method of constructing a fluid heat exchanger, the method broadly comprising the steps of constructing first and second elongate, generally flat, fluid heat exchanger tubes from an aluminum alloy, each tube having an upper and a lower surface and having a fluid inlet port and a fluid outlet port, the first tube having a series of small protrusions formed at its lower surface and the second tube having a matching series of protrusions formed at its upper surface.
Included in the construction method are the steps of cladding a brazing alloy to each of the protrusions formed on the lower surface of the first tube and to each of the protrusions formed on the upper surface of the second tube, and installing a thin sheet of a dissimilar metallic material on top of the protrusions formed at the upper surface of the second tube.
Further comprising the method are the steps of stacking the first tube on top of the sheet of dissimilar metallic material with the protrusions at the lower surface of the first tube aligned with the protrusions at the upper surface of the second tube so as to form a stacked assembly, and subjecting the stacked assembly to a temperature and for a length of time sufficient to braze the first and second tubes to the sheet of dissimilar metal.
The construction method also includes the step of connecting a fluid inlet fitting at the fluid inlet of the tubes and connecting a fluid outlet fitting at the fluid outlet of the tubes. Preferably, there is included the step of forming the protrusions at the lower surface of the first tube and the protrusions at the upper surface of the second tube by dimpling the material from which the first and second tubes are made.
Also preferably, the subjecting step includes subjecting the stacked tube assembly, in a brazing oven, to a Nocolok(copyright) brazing process temperature.
Preferably, the dissimilar metallic material is cold rolled steel having a thickness of about 0.3 millimeters. Also, the protrusions are preferably formed so as to provide a spacing of at least about 1 millimeter between major portions of the first and second heat exchanger tubes.
In accordance with an embodiment of the invention, the heat exchanger construction method comprises the steps of constructing similar, top and bottom elongate, generally flat fluid heat exchanger tubes from an aluminum alloy, the top and bottom heat exchanger tubes each having an upper and a lower surface and having a fluid inlet and a fluid outlet. The top tube has a series of small protrusions formed at its lower surface and the bottom tube has a matching series of protrusions formed at its upper surface.
The construction method further includes constructing at least one intermediate elongate, generally flat heat exchanger tube from an aluminum alloy. The at least one intermediate heat exchanger tube is similar to the top and bottom heat exchanger tubes and has an upper and a lower surface and a fluid inlet and a fluid outlet. The at least one intermediate heat exchanger tube has a series of small protrusions formed at both its upper and lower surfaces, the protrusions matching the protrusions formed on the top and bottom heat exchanger tubes.
Included are the steps of applying a brazing flux to each of the protrusions formed on the top tube, on the bottom tube and on the at least one intermediate tube, and stacking the at least one intermediate tube on top of the bottom tube and stacking the top tube on top of the at least one intermediate tube, and installing a thin sheet of a dissimilar metal between any adjacent pair of the stacked tubes on the protrusions formed on the upper surface of the tube immediately beneath the sheet.
According to the method, the step of cladding a brazing alloy to each of the protrusions formed on the lower surface of each adjacent pair of stacked tubes, and on the protrusions formed on the upper surface of the tube beneath the sheet is included, as is the step of subjecting the stacked heat exchanger tubes to a temperature and for a length of time sufficient to braze together the top tube, the bottom tube, the at least one intermediate tube and the thin sheet of dissimilar metal.
Also preferably included is the step of forming the protrusions on the top tube, the bottom tube and the at least one intermediate tube by dimpling the material from which all of tubes are made. The subjecting step preferably includes passing the stacked tubes through a brazing oven at a Nocolok(copyright) brazing process temperature.
Preferably, the protrusions are formed on the heat exchanger tubes so as to provide a spacing of at least about 1 millimeter between major portions of each adjacent pair of stacked tubes. The construction method may include installing the thin sheet of dissimilar metal on top of the bottom tube and underneath a lowermost one of the at least one intermediate tube.
An aluminum alloy heat exchanger made in accordance with the construction method comprises first and second elongate, generally flat, aluminum alloy fluid heat exchanger tubes. Each of the tubes have an upper and a lower surface and have a fluid inlet and a fluid outlet. The first tube has a series of small protrusions formed at the lower surface and the second tube has a matching series of protrusions formed at the upper surface. The first tube is stacked on top of the second tube with the protrusions on the first tube in contact with the protrusions on the second tube. The tube protrusions preferably provide a spacing of at least about 0.42 millimeter between major portions of the first and second heat exchanger tubes.
A thin sheet of a dissimilar metal is installed between the protrusions on the first and second tubes, the sheet being preferably cold rolled steel having a thickness of about 0.3 millimeters.
The heat exchanger may include a stack of an aluminum alloy top heat exchanger tube, an aluminum alloy bottom heat exchanger tube and at least one aluminum alloy intermediate heat exchanger tube. The sheet of dissimilar metal may be installed between any adjacent pair of tubes and more than one sheet of dissimilar metal mar be installed between more than one adjacent pair of stacked tubes.